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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