| Message ID | 20240405083335.507471-1-ubizjak@gmail.com (mailing list archive) |
|---|---|
| State | Accepted |
| Commit | 42698f3329f02956719bfb0db7992df8c9b9d19c |
| Headers | show |
| Series | locking/x86/xen: Use try_cmpxchg() in xen_alloc_p2m_entry() | expand |
On 05.04.24 10:32, Uros Bizjak wrote: > Use try_cmpxchg() instead of cmpxchg(*ptr, old, new) == old. > > The x86 CMPXCHG instruction returns success in the ZF flag, > so this change saves a compare after CMPXCHG. > > Also, try_cmpxchg() implicitly assigns old *ptr value to "old" > when CMPXCHG fails. There is no need to explicitly assign > old *ptr value to the temporary, which can simplify the > surrounding source code. > > No functional change intended. > > Signed-off-by: Uros Bizjak <ubizjak@gmail.com> > Cc: Juergen Gross <jgross@suse.com> > Cc: Boris Ostrovsky <boris.ostrovsky@oracle.com> > Cc: Thomas Gleixner <tglx@linutronix.de> > Cc: Ingo Molnar <mingo@kernel.org> > Cc: Borislav Petkov <bp@alien8.de> > Cc: Dave Hansen <dave.hansen@linux.intel.com> > Cc: "H. Peter Anvin" <hpa@zytor.com> Reviewed-by: Juergen Gross <jgross@suse.com> Juergen
diff --git a/arch/x86/xen/p2m.c b/arch/x86/xen/p2m.c index 9bdc3b656b2c..99918beccd80 100644 --- a/arch/x86/xen/p2m.c +++ b/arch/x86/xen/p2m.c @@ -555,7 +555,6 @@ int xen_alloc_p2m_entry(unsigned long pfn) /* Separately check the mid mfn level */ unsigned long missing_mfn; unsigned long mid_mfn_mfn; - unsigned long old_mfn; mid_mfn = alloc_p2m_page(); if (!mid_mfn) @@ -565,12 +564,12 @@ int xen_alloc_p2m_entry(unsigned long pfn) missing_mfn = virt_to_mfn(p2m_mid_missing_mfn); mid_mfn_mfn = virt_to_mfn(mid_mfn); - old_mfn = cmpxchg(top_mfn_p, missing_mfn, mid_mfn_mfn); - if (old_mfn != missing_mfn) { - free_p2m_page(mid_mfn); - mid_mfn = mfn_to_virt(old_mfn); - } else { + /* try_cmpxchg() updates missing_mfn on failure. */ + if (try_cmpxchg(top_mfn_p, &missing_mfn, mid_mfn_mfn)) { p2m_top_mfn_p[topidx] = mid_mfn; + } else { + free_p2m_page(mid_mfn); + mid_mfn = mfn_to_virt(missing_mfn); } } } else {
Use try_cmpxchg() instead of cmpxchg(*ptr, old, new) == old. The x86 CMPXCHG instruction returns success in the ZF flag, so this change saves a compare after CMPXCHG. Also, try_cmpxchg() implicitly assigns old *ptr value to "old" when CMPXCHG fails. There is no need to explicitly assign old *ptr value to the temporary, which can simplify the surrounding source code. No functional change intended. Signed-off-by: Uros Bizjak <ubizjak@gmail.com> Cc: Juergen Gross <jgross@suse.com> Cc: Boris Ostrovsky <boris.ostrovsky@oracle.com> Cc: Thomas Gleixner <tglx@linutronix.de> Cc: Ingo Molnar <mingo@kernel.org> Cc: Borislav Petkov <bp@alien8.de> Cc: Dave Hansen <dave.hansen@linux.intel.com> Cc: "H. Peter Anvin" <hpa@zytor.com> --- arch/x86/xen/p2m.c | 11 +++++------ 1 file changed, 5 insertions(+), 6 deletions(-)