linux-kernel - Re: [PATCH] FS: timerfd: Fix unexpected return value of timerfd

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Message-ID: <CACAVd4j60pn=td5hh485SJOcoYZ_jWQDQg2DVasSodPtsaupkw@mail.gmail.com>
Date:   Mon, 19 Aug 2019 20:56:49 +0530
From:   Arul Jeniston <arul.jeniston@...il.com>
To:     Thomas Gleixner <tglx@...utronix.de>
Cc:     viro@...iv.linux.org.uk, linux-fsdevel@...r.kernel.org,
        linux-kernel@...r.kernel.org, arul_mc@...l.com
Subject: Re: [PATCH] FS: timerfd: Fix unexpected return value of timerfd_read function.

hi Tglx,
> But for the above scenario:
>
> ktime_get()
>         do {
>                 seq = read_seqcount_begin(&tk_core.seq);
>                 base = tk->tkr_mono.base;
>                 nsecs = timekeeping_get_ns(&tk->tkr_mono);
>
>         } while (read_seqcount_retry(&tk_core.seq, seq));
>
> So if the interrupt which updates the timekeeper hits in the middle of
> timekeeping_get_ns() then the result is discarded because the sequence
> count changed and read_seqcount_retry() returns true. So it takes another
> round which will be perfectly aligned with the updated time keeper.
>

Do you mean to say the timekeeper  updates always happen from ktime_get?
My point was, when one thread is in ktime_get other thread/isr updates
timekeeper from different flow.

Regards,
Arul