From: Kirill Smelkov
> Sent: 26 April 2019 08:46
...
> I'm not sure I understand your comment completely, but we convert to
> stream_open only drivers that actually do _not_ use position at all, and
> that were already using nonseekable_open, thus pread and pwrite were
> already returning -ESPIPE for them (nonseekable_open clears
> FMODE_{PREAD,PWRITE} and ksys_{pread,pwrite}64 check for that flag). We
> also convert only drivers that use no_llseek for .llseek, so lseek
> on those files is/was always returning -ESPIPE as well.
>
> If a driver uses position in its read and write and has support for
> pread/pwrite (FMODE_PREAD and FMODE_PWRITE), pread and pwrite are
> already working _without_ file->f_pos locking - because those system
> calls do not semantically update file->f_pos at all and thus do not take
> file->f_pos_lock - i.e. pread/pwrite can be run simultaneously already.
Looks like I knew that once :-)
Mind you, 'man pread' on my system is somewhat uninformative.
Maybe pread() should always be allowed at offset 0.
Then you wouldn't need all this extra logic.
> If libc implements pread as lseek+read it will work for a single
> user case (single thread, or fd not shared between processes), but it
> will break because of lseek+read non-atomicity if multiple preads are
> simultaneously used from several threads. And also for such emulation
> for multiple users case there is a chance for pread vs pwrite deadlock,
> since those system calls are using read and write and read and write
> take file->f_pos_lock.
I'd actually rather the pread() failed to compile.
The actual implementation did 3 lseek()s (to save and restore the offset).
A user level emulation could usually get away with one lseek().
David
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