Re: [PATCH 2/4] Support up to 3 conditional branches in an atomic sequence

Joel Brobecker
2014-03-28T13:12:35Z

On Fri, Mar 28, 2014 at 02:41:49PM +1100, Anton Blanchard wrote:
> gdb currently supports 1 conditional branch inside a ppc larx/stcx
> critical region. Unfortunately there is existing code that contains more
> than 1, for example in the ppc64 Linux kernel:
>
> c00000000003ac18 <.__hash_page_4K>:
> ...
> c00000000003ac4c: ldarx r31,0,r6
> c00000000003ac50: andc. r0,r4,r31
> c00000000003ac54: bne- c00000000003aee8 <htab_wrong_access>
> c00000000003ac58: andi. r0,r31,2048
> c00000000003ac5c: bne- c00000000003ae0c <htab_bail_ok>
> c00000000003ac60: rlwinm r30,r4,30,24,24
> c00000000003ac64: or r30,r30,r31
> c00000000003ac68: ori r30,r30,2304
> c00000000003ac6c: oris r30,r30,4096
> c00000000003ac70: stdcx. r30,0,r6
>
> If we try to single step through this we get stuck forever because
> the reservation is never set when we step over the stdcx.
>
> The following patch bumps the number to 3 conditional branches + 1
> terminating branch. With this patch applied I can single step through
> the offending function in the ppc64 Linux kernel.
>
> gdb/
> 2014-03-28 Anton Blanchard <anton@samba.org>
>
> * breakpoint.h (MAX_SINGLE_STEP_BREAKPOINTS): New define.
> * rs6000-tdep.c (ppc_deal_with_atomic_sequence): Allow for more
> than two breakpoints.
> * breakpoint.c (insert_single_step_breakpoint): Likewise.
> (insert_single_step_breakpoint): Likewise.
> (single_step_breakpoints_inserted): Likewise.
> (cancel_single_step_breakpoints): Likewise.
> (detach_single_step_breakpoints): Likewise.
> (single_step_breakpoint_inserted_here_p): Likewise.

Overall, this looks like a nice generalization, but Pedro has been
more active in the breakpoint area than I have, so it would be nice
to have his feedback on this patch as well.

IIUC, it looks like MAX_SINGLE_STEP_BREAKPOINTS is actually not
the max, but MAX - 1. I was a little confused by that. Why not
change MAX to 3, and then adjust the array definition and code
accordingly? I think things will be slightly simpler to understand.

> ---
> gdb/breakpoint.c | 63 ++++++++++++++++++++++++++++---------------------------
> gdb/breakpoint.h | 6 ++++--
> gdb/rs6000-tdep.c | 46 ++++++++++++++++++++++------------------
> 3 files changed, 62 insertions(+), 53 deletions(-)
>
> diff --git a/gdb/breakpoint.c b/gdb/breakpoint.c
> index 19af9df..b12ea94 100644
> --- a/gdb/breakpoint.c
> +++ b/gdb/breakpoint.c
> @@ -15036,11 +15036,10 @@ deprecated_remove_raw_breakpoint (struct gdbarch *gdbarch, void *bp)
> return ret;
> }
>
> -/* One (or perhaps two) breakpoints used for software single
> - stepping. */
> +/* Breakpoints used for software single stepping. */
>
> -static void *single_step_breakpoints[2];
> -static struct gdbarch *single_step_gdbarch[2];
> +static void *single_step_breakpoints[MAX_SINGLE_STEP_BREAKPOINTS];
> +static struct gdbarch *single_step_gdbarch[MAX_SINGLE_STEP_BREAKPOINTS];
>
> /* Create and insert a breakpoint for software single step. */
>
> @@ -15049,19 +15048,17 @@ insert_single_step_breakpoint (struct gdbarch *gdbarch,
> struct address_space *aspace,
> CORE_ADDR next_pc)
> {
> + int i;
> void **bpt_p;
>
> - if (single_step_breakpoints[0] == NULL)
> - {
> - bpt_p = &single_step_breakpoints[0];
> - single_step_gdbarch[0] = gdbarch;
> - }
> - else
> - {
> - gdb_assert (single_step_breakpoints[1] == NULL);
> - bpt_p = &single_step_breakpoints[1];
> - single_step_gdbarch[1] = gdbarch;
> - }
> + for (i = 0; i < MAX_SINGLE_STEP_BREAKPOINTS; i++)
> + if (single_step_breakpoints[i] == NULL)
> + break;
> +
> + gdb_assert (i < MAX_SINGLE_STEP_BREAKPOINTS);
> +
> + bpt_p = &single_step_breakpoints[i];
> + single_step_gdbarch[i] = gdbarch;
>
> /* NOTE drow/2006-04-11: A future improvement to this function would
> be to only create the breakpoints once, and actually put them on
> @@ -15082,8 +15079,13 @@ insert_single_step_breakpoint (struct gdbarch *gdbarch,
> int
> single_step_breakpoints_inserted (void)
> {
> - return (single_step_breakpoints[0] != NULL
> - || single_step_breakpoints[1] != NULL);
> + int i;
> +
> + for (i = 0; i < MAX_SINGLE_STEP_BREAKPOINTS; i++)
> + if (single_step_breakpoints[i] != NULL)
> + return 1;
> +
> + return 0;
> }
>
> /* Remove and delete any breakpoints used for software single step. */
> @@ -15091,22 +15093,21 @@ single_step_breakpoints_inserted (void)
> void
> remove_single_step_breakpoints (void)
> {
> + int i;
> +
> gdb_assert (single_step_breakpoints[0] != NULL);
>
> /* See insert_single_step_breakpoint for more about this deprecated
> call. */
> - deprecated_remove_raw_breakpoint (single_step_gdbarch[0],
> - single_step_breakpoints[0]);
> - single_step_gdbarch[0] = NULL;
> - single_step_breakpoints[0] = NULL;
>
> - if (single_step_breakpoints[1] != NULL)
> - {
> - deprecated_remove_raw_breakpoint (single_step_gdbarch[1],
> - single_step_breakpoints[1]);
> - single_step_gdbarch[1] = NULL;
> - single_step_breakpoints[1] = NULL;
> - }
> + for (i = 0; i < MAX_SINGLE_STEP_BREAKPOINTS; i++)
> + if (single_step_breakpoints[i] != NULL)
> + {
> + deprecated_remove_raw_breakpoint (single_step_gdbarch[i],
> + single_step_breakpoints[i]);
> + single_step_gdbarch[i] = NULL;
> + single_step_breakpoints[i] = NULL;
> + }
> }
>
> /* Delete software single step breakpoints without removing them from
> @@ -15119,7 +15120,7 @@ cancel_single_step_breakpoints (void)
> {
> int i;
>
> - for (i = 0; i < 2; i++)
> + for (i = 0; i < MAX_SINGLE_STEP_BREAKPOINTS; i++)
> if (single_step_breakpoints[i])
> {
> xfree (single_step_breakpoints[i]);
> @@ -15136,7 +15137,7 @@ detach_single_step_breakpoints (void)
> {
> int i;
>
> - for (i = 0; i < 2; i++)
> + for (i = 0; i < MAX_SINGLE_STEP_BREAKPOINTS; i++)
> if (single_step_breakpoints[i])
> target_remove_breakpoint (single_step_gdbarch[i],
> single_step_breakpoints[i]);
> @@ -15151,7 +15152,7 @@ single_step_breakpoint_inserted_here_p (struct address_space *aspace,
> {
> int i;
>
> - for (i = 0; i < 2; i++)
> + for (i = 0; i < MAX_SINGLE_STEP_BREAKPOINTS; i++)
> {
> struct bp_target_info *bp_tgt = single_step_breakpoints[i];
> if (bp_tgt
> diff --git a/gdb/breakpoint.h b/gdb/breakpoint.h
> index d8e88fc..65d3ecb 100644
> --- a/gdb/breakpoint.h
> +++ b/gdb/breakpoint.h
> @@ -1443,8 +1443,10 @@ extern void add_solib_catchpoint (char *arg, int is_load, int is_temp,
> deletes all breakpoints. */
> extern void delete_command (char *arg, int from_tty);
>
> -/* Manage a software single step breakpoint (or two). Insert may be
> - called twice before remove is called. */
> +/* Manage software single step breakpoints. Insert may be
> + called multiple times before remove is called. */
> +#define MAX_SINGLE_STEP_BREAKPOINTS 4
> +
> extern void insert_single_step_breakpoint (struct gdbarch *,
> struct address_space *,
> CORE_ADDR);
> diff --git a/gdb/rs6000-tdep.c b/gdb/rs6000-tdep.c
> index dbe3aa2..be14e39 100644
> --- a/gdb/rs6000-tdep.c
> +++ b/gdb/rs6000-tdep.c
> @@ -1088,7 +1088,7 @@ ppc_deal_with_atomic_sequence (struct frame_info *frame)
> struct address_space *aspace = get_frame_address_space (frame);
> enum bfd_endian byte_order = gdbarch_byte_order (gdbarch);
> CORE_ADDR pc = get_frame_pc (frame);
> - CORE_ADDR breaks[2] = {-1, -1};
> + CORE_ADDR breaks[MAX_SINGLE_STEP_BREAKPOINTS];
> CORE_ADDR loc = pc;
> CORE_ADDR closing_insn; /* Instruction that closes the atomic sequence. */
> int insn = read_memory_integer (loc, PPC_INSN_SIZE, byte_order);
> @@ -1097,7 +1097,6 @@ ppc_deal_with_atomic_sequence (struct frame_info *frame)
> int last_breakpoint = 0; /* Defaults to 0 (no breakpoints placed). */
> const int atomic_sequence_length = 16; /* Instruction sequence length. */
> int opcode; /* Branch instruction's OPcode. */
> - int bc_insn_count = 0; /* Conditional branch instruction count. */
>
> /* Assume all atomic sequences start with a lwarx/ldarx instruction. */
> if ((insn & LWARX_MASK) != LWARX_INSTRUCTION
> @@ -1111,24 +1110,20 @@ ppc_deal_with_atomic_sequence (struct frame_info *frame)
> loc += PPC_INSN_SIZE;
> insn = read_memory_integer (loc, PPC_INSN_SIZE, byte_order);
>
> - /* Assume that there is at most one conditional branch in the atomic
> - sequence. If a conditional branch is found, put a breakpoint in
> - its destination address. */
> if ((insn & BRANCH_MASK) == BC_INSN)
> {
> int immediate = ((insn & 0xfffc) ^ 0x8000) - 0x8000;
> int absolute = insn & 2;
>
> - if (bc_insn_count >= 1)
> - return 0; /* More than one conditional branch found, fallback
> + if (last_breakpoint >= MAX_SINGLE_STEP_BREAKPOINTS - 1)
> + return 0; /* too many conditional branches found, fallback
> to the standard single-step code. */
>
> if (absolute)
> - breaks[1] = immediate;
> + breaks[last_breakpoint] = immediate;
> else
> - breaks[1] = loc + immediate;
> + breaks[last_breakpoint] = loc + immediate;
>
> - bc_insn_count++;
> last_breakpoint++;
> }
>
> @@ -1147,18 +1142,29 @@ ppc_deal_with_atomic_sequence (struct frame_info *frame)
> insn = read_memory_integer (loc, PPC_INSN_SIZE, byte_order);
>
> /* Insert a breakpoint right after the end of the atomic sequence. */
> - breaks[0] = loc;
> + breaks[last_breakpoint] = loc;
>
> - /* Check for duplicated breakpoints. Check also for a breakpoint
> - placed (branch instruction's destination) anywhere in sequence. */
> - if (last_breakpoint
> - && (breaks[1] == breaks[0]
> - || (breaks[1] >= pc && breaks[1] <= closing_insn)))
> - last_breakpoint = 0;
> -
> - /* Effectively inserts the breakpoints. */
> for (index = 0; index <= last_breakpoint; index++)
> - insert_single_step_breakpoint (gdbarch, aspace, breaks[index]);
> + {
> + int index2;
> + int insert_bp = 1;
> +
> + /* Check for a breakpoint placed (branch instruction's destination)
> + anywhere in sequence. */
> + if (breaks[index] >= pc && breaks[index] <= closing_insn)
> + continue;
> +
> + /* Check for duplicated breakpoints. */
> + for (index2 = 0; index2 < index; index2++)
> + {
> + if (breaks[index] == breaks[index2])
> + insert_bp = 0;
> + }
> +
> + /* insert the breakpoint. */
> + if (insert_bp)
> + insert_single_step_breakpoint (gdbarch, aspace, breaks[index]);
> + }
>
> return 1;
> }
> --
> 1.8.3.2

--
Joel