Re: (stat(...) == -1 || faccessat(...) == -1) && errno == EINTR ?!??

[Tobias Bading <tbading@web.de>]

o/

I ran the test program against different servers and shares in the
office. The share on which I encountered the problem first seems to be
affected the worst: 50 SIGALRMs per second and the first stat() has no
chance. Other shares, some on Windows servers and one on an older
GNU/Linux with Samba, seemed not to have this problem at first. But when
I removed the teminal output from the SIGALRM handler and increased the
SIGALRM rate to 10000 per second, all those other shares produced the
error as well sooner or later.

Hmm...

Tobias

On 14.02.21 13:18, Tobias Bading wrote:
> Hello again.
>
> I've been able to reproduce the problem with the attached program.
> With SIGALRMs firing 10 times per second, I get maybe a dozen "handler
> called" lines before stat() or faccessat() fails with errno EINTR.
> When I increase the rate to 50 SIGALRMs per second, the very first
> stat() fails in every test run.
>
> Specifying SA_RESTART in sigaction() has no effect.
>
> Unfortunately, I don't have any other network shares available at the
> moment to test whether only CIFS through VPN is affected, or the
> problem would occur with e.g. NFS or CIFS without a VPN as well.
>
> Tobias
>
> ---
>
> On 14.02.21 12:13, Tobias Bading wrote:
>> Hi.
>>
>> A few days ago I encountered strange problems at home while using GNU
>> Emacs on GNU/Linux with Windows SMB shares in the office automounted
>> through the company's VPN.
>>
>> Details and my current findings on the Emacs devel mailing list:
>> https://lists.gnu.org/archive/html/emacs-devel/2021-02/msg00835.html.
>>
>> TL;DR:
>> Emacs' stat() and faccessat() calls sometimes return -1 and set errno
>> to EINTR (according to strace). Most of the time the interruption
>> seems to come from a SIGIO, but at least once a SIGCHLD was in the
>> mix as well.
>>
>> The behavior of returning with errno == EINTR seems to be new and/or
>> some kind of bug, because otherwise Emacs would handle that case with
>> TEMP_FAILURE_RETRY() or something to that effect, right? I've used
>> pretty much the same Emacs in the office (on an older GNU/Linux
>> release) for ages with the same Windows SMB shares and never had this
>> problem.
>>
>> The GNU/Linux man pages of stat() and faccessat() don't mention EINTR
>> at all, and neither does POSIX
>> (https://pubs.opengroup.org/onlinepubs/9699919799/).
>>
>> So I guess the one million dollar question is this:
>> Are stat() and faccessat() permitted to return -1 with errno ==
>> EINTR, or does that violate the POSIX (or some other) spec?
>>
>> I'll probably write a small test program next to check if I can
>> reproduce this problem outside of Emacs, maybe with a simple SIGALRM
>> and an endless loop of stat()/faccessat() calls. If that reproduces
>> the problem it would be nice to know whether using SA_RESTART in
>> sigaction() has any effect.
>>
>> Any ideas are very welcome... :)
>>
>> Tobias
>>
>> PS: I'm running Ubuntu focal (20.04.2 LTS, amd64), which currently
>> uses libc-bin 2.31-0ubuntu9.2, kernel linux-image-5.4.0-64-generic
>> and openconnect 8.05-1 to access the VPN.
>>
>