pipe writes, ERESTARTSYS and SA_RESTART

[Yann Droneaud]

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

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