Messages in this thread | | | Date | Fri, 8 Feb 2013 01:13:13 +0400 | Subject | Re: [PATCH 3/3] signalfd: add ability to read siginfo-s without dequeuing signals (v2) | From | Andrey Wagin <> |
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2013/2/7 Oleg Nesterov <oleg@redhat.com>: > Andrey, sorry for delay. > > As for API, I leave this to you and Michael. Not that I like these > new flags, but I agree that pread() hack was not pretty too. > > On 01/29, Andrey Vagin wrote: >> +static ssize_t signalfd_peek(struct signalfd_ctx *ctx, >> + siginfo_t *info, loff_t *ppos, int queue_mask) >> +{ >> + loff_t seq = *ppos / sizeof(struct signalfd_siginfo); >> + int signr = 0; >> + >> + if (queue_mask & SIGQUEUE_PRIVATE) >> + signr = peek_signal(¤t->pending, >> + &ctx->sigmask, info, &seq); >> + else if (queue_mask & SIGQUEUE_SHARED) >> + signr = peek_signal(¤t->signal->shared_pending, >> + &ctx->sigmask, info, &seq); >> + (*ppos) += sizeof(struct signalfd_siginfo); > > Now that this can work even with normal read(), we will actually change > f_pos. Then perhaps signalfd_fops->llseek() should work too. But this > is minor...
lseek works only if FMODE_LSEEK is set.
You have explained why read&lseek have strange semantics for SIGNALFD_PEEK.
>Damn. But after I wrote this email I realized that llseek() probably can't > work. Because peek_offset/f_pos/whatever has to be shared with all processes > which have this file opened. > > Suppose that the task forks after sys_signalfd(). Now if parent or child > do llseek this affects them both. This is insane because signalfd is > "strange" to say at least, fork/dup/etc inherits signalfd_ctx but not the > "source" of the data.
So I want to suggest a way how to forbid read() for SIGNALFD_PEEK. file->f_pos can be initialized to -1. read() returns EINVAL in this case. In a man page we will write that signals can be dumped only with help pread(). Is it overload or too ugly?
> > Hmm. but since it works with read(), we shouldn't increment *ppos unless > signalfd_copyinfo() succeeds?
No, we shouldn't.
> > Btw, why do you pass seq by reference? Looks unneeded.
You are right. I created this code for reading signals from both queues, but then we decided to forbid using SIGNALFD_PEEK for both queues simultaneously.
Oleg, thank you for the comments. I'm waiting an answer on the question and after that I'm going to send a final version.
> > Oleg. >
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