Re: [PATCH] VFS: make file->f_pos access atomic on 32bit arch

[Michael Trimarchi]

Hi,

----- Messaggio originale -----
> Da: Nick Piggin <nickpiggin@xxxxxxxxxxxx>
> A: Michael Trimarchi <trimarchimichael@xxxxxxxx>
>
Cc: Hisashi Hifumi <hifumi.hisashi@xxxxxxxxxxxxx>;
akpm@xxxxxxxxxxxxxxxxxxxx; linux-kernel@xxxxxxxxxxxxxxx;
linux-fsdevel@xxxxxxxxxxxxxxx
> Inviato: Martedì 16 settembre 2008, 16:11:21
> Oggetto: Re: [PATCH] VFS: make file->f_pos access atomic on 32bit arch
> 
...
> >  i was thinking about introducing a file_pos_update() not implemented using
> > file_pos_read()/file_pos_write() and taking the seq_lock or
> > disabling preemption only one time
> 
> AFAIK, the only way to do an atomic 64 bit store on 32-bit x86 is
> "cmpxchg8b" (with lock prefix on SMP). That would be far slower
> I'm sure.

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

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