From: Adhemerval Zanella <adhemerval.zanella@linaro.org>
To: Zack Weinberg <zackw@panix.com>
Cc: GNU C Library <libc-alpha@sourceware.org>
Subject: Re: [PATCH v2 02/21] nptl: Fix testcases for new pthread cancellation mechanism
Date: Tue, 8 May 2018 14:26:00 -0300 [thread overview]
Message-ID: <96d3f55e-f307-f9aa-61df-a5057215b959@linaro.org> (raw)
In-Reply-To: <CAKCAbMgnO_p-P37_67MuQDLqhtTQLGud2bo-MUzvdq+yBnu8fw@mail.gmail.com>
On 08/05/2018 10:35, Zack Weinberg wrote:
> On Mon, May 7, 2018 at 1:13 PM, Adhemerval Zanella
> <adhemerval.zanella@linaro.org> wrote:
>> On 06/05/2018 23:49, Zack Weinberg wrote:
>>> On 26 Feb 2018, Adhemerval Zanella wrote:
>>>> For tst-cancel{2,3} case it remove the pipe close because it might
>>>> cause the write syscall to return with side effects if the close is
>>>> executed before the pthread cancel.
>>>
>>> ... however, this change appears to be wrong. If cancellation is
>>> broken, these tests will now deadlock rather than failing cleanly.
>>
>> On current cancellation implementation the thread will finish regardless
>> and sigcancel_handler will act whether there is side-effects or not
>> (the pipe close). The issue is cancellation should not happen if syscall
>> returns but some side effects already took place, in this case the pipe
>> close.
>
> I think maybe I didn't explain clearly enough what I'm worried about
> here. What the test case does _when cancellation works_ is sensible.
> But this is a test case, it also needs to behave sensibly when
> cancellation _doesn't_ work. Imagine a new port where, for some
> reason, the cancellation mechanism is so broken that read/write aren't
> acting as cancellation points at all. Without the close,
> tst-cancel{2,3} will block forever in read/write. We have the
> test-driver timeout as a backstop, but we shouldn't rely on it.
Well, adding a timing mechanism (either by alarm, pthread_timedjoin, etc)
is basically what the test-driver backstop is actually doing, with the
advantage it is another process no susceptible to possible memory
corruptions or other execution failures. I don't see much gain in
add more hardening in test itself.
>
>> Yes, although for this specific case I am not sure if this could happen
>> in practice. I assume if a thread issues a 'signal' followed by a 'close',
>> the signal target thread will receive the events in a ordered manner, i.e,
>> the signal handler will be activated before the syscall sees any
>> side-effects (the close). It seems to be Linux behaviour, but I am not
>> sure if a different system might act differently.
>
> I don't think POSIX makes any requirements, but yes, in practice the
> signal should always arrive first.
>
>> And I try to avoid the timing check, such as pthread_timedjoin_np,
>> because they tend to quite fragile in practice for such cases (due either
>> to system load when testing glibc, machine performance, etc.).
>
> This is reasonable.
>
> For the new cancellation mechanism in general, we don't have a good
> way of arranging for SIGCANCEL to arrive at exactly the critical
> points within the syscall sequence, do we? I am tempted to try to
> write a test case that scripts gdb and single-steps through a call to
> open() and fires SIGCANCEL at each instruction.
I can't really think any explicit way to actually check it, a gdb/ptrace
is the close think I can think off as well.
>
>>> won't it? I think teaching the backtrace logic about this would be
>>> better than needing to use a raw syscall() and then mess with the
>>> PowerPC implementation of syscall(). I might feel differently about
>>> this change if __read_nocancel were a public API, but it isn't...
>>
>> With your current suggestion to powerpc syscall bits, there is no need
>> to actually change the powerpc syscall implementation besides an
>> additional CFI mechanism. But I do not mind to change the testcase on
>> the bz12683 fix itself, the only advantage I see is by using indirect
>> syscall there is no need to actually change it again.
>
> I don't feel especially strongly about this now we have a way that
> doesn't add actual instructions to powerpc syscall().
>
> zw
>
Right, I will my current version on next version.
next prev parent reply other threads:[~2018-05-08 17:26 UTC|newest]
Thread overview: 45+ messages / expand[flat|nested] mbox.gz Atom feed top
2018-02-26 21:03 [PATCH v2 00/21] nptl: Fix Race conditions in pthread cancellation (BZ#12683) Adhemerval Zanella
2018-02-26 21:03 ` [PATCH v2 01/21] powerpc: Create stackframe information on syscall Adhemerval Zanella
2018-02-26 21:41 ` Andreas Schwab
2018-02-27 12:05 ` Adhemerval Zanella
2018-05-07 2:49 ` Zack Weinberg
2018-05-07 13:57 ` Adhemerval Zanella
2018-02-26 21:03 ` [PATCH v2 02/21] nptl: Fix testcases for new pthread cancellation mechanism Adhemerval Zanella
2018-02-26 21:43 ` Andreas Schwab
2018-02-27 12:05 ` Adhemerval Zanella
2018-05-07 2:49 ` Zack Weinberg
2018-05-07 17:13 ` Adhemerval Zanella
2018-05-08 13:35 ` Zack Weinberg
2018-05-08 17:26 ` Adhemerval Zanella [this message]
2018-02-26 21:03 ` [PATCH v2 03/21] nptl: Fix Race conditions in pthread cancellation (BZ#12683) Adhemerval Zanella
2018-04-27 12:20 ` Zack Weinberg
2018-04-27 12:25 ` Adhemerval Zanella
2018-05-07 2:48 ` Zack Weinberg
2018-05-07 2:49 ` Zack Weinberg
2018-05-08 17:11 ` Adhemerval Zanella
2018-02-26 21:03 ` [PATCH v2 04/21] nptl: x86_64: " Adhemerval Zanella
2018-05-07 2:49 ` Zack Weinberg
2018-05-08 17:55 ` Adhemerval Zanella
2018-02-26 21:03 ` [PATCH v2 05/21] nptl: x32: " Adhemerval Zanella
2018-02-26 21:03 ` [PATCH v2 06/21] nptl: i386: " Adhemerval Zanella
2018-05-07 2:49 ` Zack Weinberg
2018-05-08 17:56 ` Adhemerval Zanella
2018-02-26 21:03 ` [PATCH v2 07/21] nptl: powerpc: " Adhemerval Zanella
2018-05-07 19:25 ` Tulio Magno Quites Machado Filho
2018-05-08 18:07 ` Adhemerval Zanella
2018-02-26 21:03 ` [PATCH v2 08/21] nptl: aarch64: " Adhemerval Zanella
2018-02-26 21:03 ` [PATCH v2 09/21] nptl: arm: " Adhemerval Zanella
2018-02-26 21:03 ` [PATCH v2 10/21] nptl: s390: " Adhemerval Zanella
2018-02-26 21:03 ` [PATCH v2 11/21] nptl: ia64: " Adhemerval Zanella
2018-02-26 21:03 ` [PATCH v2 12/21] nptl: alpha: " Adhemerval Zanella
2018-02-26 21:03 ` [PATCH v2 13/21] nptl: m68k: " Adhemerval Zanella
2018-02-26 21:03 ` [PATCH v2 14/21] nptl: microblaze: " Adhemerval Zanella
2018-02-26 21:03 ` [PATCH v2 15/21] nptl: tile: " Adhemerval Zanella
2018-02-26 21:03 ` [PATCH v2 16/21] nptl: sparc: " Adhemerval Zanella
2018-02-26 21:03 ` [PATCH v2 17/21] nptl: nios2: " Adhemerval Zanella
2018-02-26 21:03 ` [PATCH v2 18/21] nptl: sh: " Adhemerval Zanella
2018-02-26 21:03 ` [PATCH v2 19/21] nptl: mips: " Adhemerval Zanella
2018-02-26 21:03 ` [PATCH v2 20/21] nptl: hppa: " Adhemerval Zanella
2018-02-26 21:03 ` [PATCH v2 21/21] nptl: riscv: " Adhemerval Zanella
2018-02-27 1:16 ` DJ Delorie
2018-02-27 13:03 ` Adhemerval Zanella
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