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Re: [PATCH v2 2/3] PR remote/19496, interrupted syscall in forking-threads-plus-bkpt
- From: Don Breazeal <donb at codesourcery dot com>
- To: <gdb-patches at sourceware dot org>, <palves at redhat dot com>
- Date: Thu, 25 Feb 2016 09:28:47 -0800
- Subject: Re: [PATCH v2 2/3] PR remote/19496, interrupted syscall in forking-threads-plus-bkpt
- Authentication-results: sourceware.org; auth=none
- References: <1455150484-12600-1-git-send-email-donb at codesourcery dot com>
Ping
Thanks,
--Don
On 2/10/2016 4:28 PM, Don Breazeal wrote:
> Hi Pedro,
>
> On 2/1/2016 11:38 AM, Pedro Alves wrote:
>> On 01/28/2016 12:48 AM, Don Breazeal wrote:
>>> This patch addresses "fork:Interrupted system call" (or wait:) failures
>>> in gdb.threads/forking-threads-plus-breakpoint.exp.
>>>
>>> The test program spawns ten threads, each of which do ten fork/waitpid
>>> sequences. The cause of the problem was that when one of the fork
>>> children exited before the corresponding fork parent could initiate its
>>> waitpid for that child, a SIGCHLD was delivered and interrupted a fork
>>> or waitpid in another thread.
>
> In fact, I think my diagnosis here was incorrect, or at least incorrect
> in some cases. I believe at least some of the interruptions are caused
> by SIGSTOP, sent by GDB when stopping all the threads. More below.
>
>>>
>>> The fix was to wrap the system calls in a loop to retry the call if
>>> it was interrupted, like:
>>>
>>> do
>>> {
>>> pid = fork ();
>>> }
>>> while (pid == -1 && errno == EINTR);
>>>
>>> Since this is a Linux-only test I figure it is OK to use errno and EINTR.
>>>
>>> Tested on Nios II Linux target with x86 Linux host.
>>
>> I'd prefer to avoid this if possible. These loops potentially hide
>> bugs like ERESTARTSYS escaping out of a syscall and mishandling of
>> signals. See bc9540e842eb5639ca59cb133adef211d252843c for example:
>> https://sourceware.org/ml/gdb-patches/2015-02/msg00654.html
>>
>> How about setting SIGCHLD to SIG_IGN, or making SIGCHLD be SA_RESTART?
>
> I spent a couple of days trying to find an alternate solution, but
> couldn't find one that was reliable. Here is a snapshot of what I tried:
>
> 1) SIG_IGN: results in an ECHILD from waitpid. The man page for waitpid
> says "This can happen for one's own child if the action for SIGCHLD is
> set to SIG_IGN."
>
> 2) SA_RESTART: While waitpid is listed as a system call that can be
> restarted by SA_RESTART, fork is not. Even if I leave the "EINTR loop"
> in place for fork, using SA_RESTART I still see an interrupted system
> call for waitpid. Possibly because the problem is SIGSTOP and not
> SIGCHLD.
>
> 3) pthread_sigblock: With this set for SIGCHLD in all the threads, I
> still saw an interrupted system call. You can't block SIGSTOP.
>
> 4) pthread_sigblock with sigwait: using pthread_sigblock on all the
> blockable signals with a signal thread that called sigwait for all
> the signals in a loop, the signal thread would see a bunch of SIGCHLDs,
> but there would eventually be an interrupted system call.
>
> 5) bsd_signal: this function is supposed to automatically restart blocking
> system calls. fork is not a blocking system call, but it doesn't help
> for waitpid either.
>
> I found this in the ptrace(2) man page: "Note that a suppressed signal
> still causes system calls to return prematurely. In this case, system
> calls will be restarted: the tracer will observe the tracee to reexecute
> the interrupted system call (or restart_syscall(2) system call for a few
> system calls which use a different mechanism for restarting) if the tracer
> uses PTRACE_SYSCALL. Even system calls (such as poll(2)) which are not
> restartable after signal are restarted after signal is suppressed; however,
> kernel bugs exist which cause some system calls to fail with EINTR even
> though no observable signal is injected to the tracee."
>
> The GDB manual mentions something similar about interrupted system calls.
>
> So, the bottom line is that I haven't changed the fix for the interrupted
> system calls, because I can't find anything that works as well as the
> original fix. Perhaps this test puts enough stress on the kernel that the
> kernel bugs mentioned above are exposed.
>
> One change I did make from the previous version was to increase the
> timeout to 90 seconds, which was necessary to get more reliable results
> on the Nios II target.
>
> Let me know what you think.
> Thanks!
> --Don
>
> ---
> This patch addresses "fork:Interrupted system call" (or wait:) failures
> in gdb.threads/forking-threads-plus-breakpoint.exp.
>
> The test program spawns ten threads, each of which do ten fork/waitpid
> sequences. The cause of the problem was that when one of the fork
> children exited before the corresponding fork parent could initiate its
> waitpid for that child, a SIGCHLD was delivered and interrupted a fork
> or waitpid in another thread.
>
> The fix was to wrap the system calls in a loop to retry the call if
> it was interrupted, like:
>
> do
> {
> pid = fork ();
> }
> while (pid == -1 && errno == EINTR);
>
> Since this is a Linux-only test I figure it is OK to use errno and EINTR.
> I tried a number of alternative fixes using SIG_IGN, SA_RESTART,
> pthread_sigblock, and bsd_signal, but none of these worked as well.
>
> Tested on Nios II Linux target with x86 Linux host.
>
> gdb/testsuite/ChangeLog:
> 2016-02-10 Don Breazeal <donb@codesourcery.com>
>
> * gdb.threads/forking-threads-plus-breakpoint.c (thread_forks):
> Retry fork and waitpid on interrupted system call errors.
> * gdb.threads/forking-threads-plus-breakpoint.exp: (do_test):
> Increase timeout to 90.
>
> ---
> .../gdb.threads/forking-threads-plus-breakpoint.c | 14 ++++++++++++--
> .../gdb.threads/forking-threads-plus-breakpoint.exp | 3 +++
> 2 files changed, 15 insertions(+), 2 deletions(-)
>
> diff --git a/gdb/testsuite/gdb.threads/forking-threads-plus-breakpoint.c b/gdb/testsuite/gdb.threads/forking-threads-plus-breakpoint.c
> index fc64d93..c169e18 100644
> --- a/gdb/testsuite/gdb.threads/forking-threads-plus-breakpoint.c
> +++ b/gdb/testsuite/gdb.threads/forking-threads-plus-breakpoint.c
> @@ -22,6 +22,7 @@
> #include <sys/types.h>
> #include <sys/wait.h>
> #include <stdlib.h>
> +#include <errno.h>
>
> /* Number of threads. Each thread continuously spawns a fork and wait
> for it. If we have another thread continuously start a step over,
> @@ -49,14 +50,23 @@ thread_forks (void *arg)
> {
> pid_t pid;
>
> - pid = fork ();
> + do
> + {
> + pid = fork ();
> + }
> + while (pid == -1 && errno == EINTR);
>
> if (pid > 0)
> {
> int status;
>
> /* Parent. */
> - pid = waitpid (pid, &status, 0);
> + do
> + {
> + pid = waitpid (pid, &status, 0);
> + }
> + while (pid == -1 && errno == EINTR);
> +
> if (pid == -1)
> {
> perror ("wait");
> diff --git a/gdb/testsuite/gdb.threads/forking-threads-plus-breakpoint.exp b/gdb/testsuite/gdb.threads/forking-threads-plus-breakpoint.exp
> index ff3ca9a..6889c2b 100644
> --- a/gdb/testsuite/gdb.threads/forking-threads-plus-breakpoint.exp
> +++ b/gdb/testsuite/gdb.threads/forking-threads-plus-breakpoint.exp
> @@ -73,6 +73,9 @@ proc do_test { cond_bp_target detach_on_fork displaced } {
> global linenum
> global is_remote_target
>
> + global timeout
> + set timeout 90
> +
> set saved_gdbflags $GDBFLAGS
> set GDBFLAGS [concat $GDBFLAGS " -ex \"set non-stop on\""]
> clean_restart $binfile
>