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Message-Id: <6.0.0.20.2.20080918153651.04e39b70@172.19.0.2>
Date: Thu, 18 Sep 2008 16:08:35 +0900
From: Hisashi Hifumi <hifumi.hisashi@....ntt.co.jp>
To: Michael Trimarchi <trimarchimichael@...oo.it>,
Nick Piggin <nickpiggin@...oo.com.au>,
akpm@...ux-foundation.org
Cc: linux-kernel@...r.kernel.org, linux-fsdevel@...r.kernel.org
Subject: Re: [PATCH] VFS: make file->f_pos access atomic on 32bit arch
>
>Maybe it is clear with an example:
>
>+/*
>+ * file_pos_read/write is atomic by using sequence lock on 32bit arch.
>+ */
>+static inline loff_t file_pos_read(struct file *file)
>+{
>+#if BITS_PER_LONG == 32 && defined(CONFIG_SMP)
>+ loff_t pos;
>+ unsigned int seq;
>+
>+ do {
>+ seq = read_seqbegin(&file->f_pos_seqlock);
>+ pos = file->f_pos;
>+ } while (read_seqretry(&file->f_pos_seqlock, seq));
>+ return pos;
>+#elif BITS_PER_LONG == 32 && defined(CONFIG_PREEMPT)
>+ loff_t pos;
>+
>+ preempt_disable();
>+ pos = file->f_pos;
>+ preempt_enable();
>+ return pos;
>+#else
>+ return file->f_pos;
>+#endif
>+}
>+
>+static inline void file_pos_write(struct file *file, loff_t pos)
>+{
>+#if BITS_PER_LONG == 32 && defined(CONFIG_SMP)
>+ write_seqlock(&file->f_pos_seqlock);
>+ file->f_pos = pos;
>+ write_sequnlock(&file->f_pos_seqlock);
>+#elif BITS_PER_LONG == 32 && defined(CONFIG_PREEMPT)
>+ preempt_disable();
>+ file->f_pos = pos;
>+ preempt_enable();
>+#else
>+ file->f_pos = pos;
>+#endif
>+}
>+
>if (offset != file_pos_read(file))
> file_pos_write(file, offset);
>
>Compile with:
>BITS_PER_LONG=32
>CONFIG_PREEMPT
>
>The code does:
>
>file_pos_read(...)
>preempt_disable
>....
>preempt_enable
>
>file_pos_write(...)
>preempt_disable
>....
>preempt_enable
>
>with the file_pos_update() the code does:
>
>preempt_disable
>...
>...
>preempt_enable
>
>Regards Michael
>
I think file_pos_update() with BITS_PER_LONG=32 && CONFIG_PREEMPT is easy,
this is like this.
static inline void file_pos_update(struct file *file, loff_t pos)
{
preempt_disable();
if (pos != file->f_pos)
file->f_pos = pos;
preempt_enable();
}
But I think BITS_PER_LONG=32 && CONFIG_SMP version has a problem, this is like this.
static inline void file_pos_update(struct file *file, loff_t pos)
{
write_seqlock(&file->f_pos_seqlock);
if (pos != file->f_pos)
file->f_pos = pos;
write_sequnlock(&file->f_pos_seqlock);
}
file_pos_update acquires seqlock only once, but write_seqlock is called whether file->f_pos
is overwritten or not.
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