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Re: [Xen-devel] [PATCH v2] rwlock: allow recursive read locking when already locked in write mode

On 21/02/2020 14:35, Jürgen Groß wrote:
On 21.02.20 15:32, Julien Grall wrote:

On 21/02/2020 14:16, Jürgen Groß wrote:
On 21.02.20 15:11, Julien Grall wrote:
Hi Juergen,

On 21/02/2020 14:06, Jürgen Groß wrote:
On 21.02.20 14:49, Julien Grall wrote:

On 21/02/2020 13:46, Jürgen Groß wrote:
On 21.02.20 14:36, Jan Beulich wrote:
On 21.02.2020 10:10, Roger Pau Monné wrote:
On Thu, Feb 20, 2020 at 07:20:06PM +0000, Julien Grall wrote:

On 20/02/2020 17:31, Roger Pau Monne wrote:
Allow a CPU already holding the lock in write mode to also lock it in read mode. There's no harm in allowing read locking a rwlock that's already owned by the caller (ie: CPU) in write mode. Allowing such
accesses is required at least for the CPU maps use-case.

In order to do this reserve 14bits of the lock, this allows to support up to 16384 CPUs. Also reduce the write lock mask to 2 bits: one to signal there are pending writers waiting on the lock and the other to signal the lock is owned in write mode. Note the write related data is using 16bits, this is done in order to be able to clear it (and
thus release the lock) using a 16bit atomic write.

This reduces the maximum number of concurrent readers from 16777216 to 65536, I think this should still be enough, or else the lock field can be expanded from 32 to 64bits if all architectures support atomic
operations on 64bit integers.

FWIW, arm32 is able to support atomic operations on 64-bit integers.

   static inline void _write_unlock(rwlock_t *lock)
-    /*
-     * If the writer field is atomic, it can be cleared directly. -     * Otherwise, an atomic subtraction will be used to clear it.
-     */
-    atomic_sub(_QW_LOCKED, &lock->cnts);
+    /* Since the writer field is atomic, it can be cleared directly. */
+    ASSERT(_is_write_locked_by_me(atomic_read(&lock->cnts)));
+    BUILD_BUG_ON(_QR_SHIFT != 16);
+    write_atomic((uint16_t *)&lock->cnts, 0);

I think this is an abuse to cast an atomic_t() directly into a uint16_t. You
would at least want to use &lock->cnts.counter here.

Sure, I was wondering about this myself.

Will wait for more comments, not sure whether this can be fixed upon
commit if there are no other issues.

It's more than just adding another field specifier here. A cast like
this one is endianness-unsafe, and hence a trap waiting for a big
endian port attempt to fall into. At the very least this should cause a build failure on big endian systems, even better would be if it was

Wouldn't a union be the better choice?

You would not be able to use atomic_t in that case as you can't assume the layout of the structure.

union rwlockword {
     atomic_t cnts;
     uint32_t val;
     struct {
         uint16_t write;
         uint16_t readers;

static inline const uint32_t _qr_bias(
     const union rwlockword {
         .write = 0,
         .readers = 1
     } x;

     return x.val;

     cnts = atomic_add_return(_qr_bias(), &lock->cnts);

I guess this should do the trick, no?

You are assuming the atomic_t layout which I would rather no want to happen.

That already happened. rwlock.h already assumes atomic_t to be 32 bits
wide. I agree it would be better to have an atomic32_t type for this
use case.

I don't think you understood my point here. An atomic32_t will not be better as you still assume the layout of the structure. I.e the structure has only one field.

I don't understand your reasoning here, sorry.

Are you saying that a structure of two uint16_t isn't known to be 32
bits long?

You are assuming that atomic_t will always be:

  int counter;

What if we decide to turn into

  bool a;
  int counter;

You would at least want a BUILD_BUG_ON() as at the compiler will throw you an error rather than happily countinuing.


Julien Grall

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