Hi,
Le 04/08/2022 à 20:01, Viacheslav Biriukov a écrit :
But what I can't understand is how and where the kernel modifies the
arguments of the write system call and where it collects the return
values of all these restarts, thus the userspace caller ultimately
sees the correct number of written bytes.
With strace I can see all that retries, for example:
write(1, ""..., 33554431) = 65536
write(1, ""..., 33488895) = ? ERESTARTSYS (To be
restarted if SA_RESTART is set)
write(1, ""..., 33488895) = ? ERESTARTSYS (To be
restarted if SA_RESTART is set)
write(1, ""..., 33488895) = ? ERESTARTSYS (To be
restarted if SA_RESTART is set)
write(1, ""..., 33488895) = 33488895
Here there were 4 restarts (I sent 4 signals), 3 of them returned
ERESTARTSYS and 2 were able to write to the pipe. Also for restarts
strace shows the correct 3rd argument, which is decrementing.
The caller in the userspace in the end sees that it was able to write
65536+33488895 bytes. Which is correct and what the man 7 pipe describes.
My question is how and where it does that. I tried to dig in the
kernel source code but can't find the place where this tracking occurs.
It doesn't. SA_RESTART is only meant to retry syscall that would have
returned EINTR.
In such case, there's no tracking to do, because nothing was actually
written, so the syscall can be restarted with the same parameters.
Regards.
--
Yann Droneaud
OPTEYA
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