On Fri, Feb 19, 2021 at 01:03:11PM -0600, Eric Blake wrote: > The only time where we have to be careful is when the client sends > unaligned data; if we add a lock around our use of the bounce buffer, > then we can provide the same parallelism as the underlying plugin for > all other transactions. > > When we first added .can_multi_conn, we waffled on what the blocksize > filter should do[1]; blindly slamming to 1 when all requests are > serial would have been correct at the time, but we opted to stay > conservative by instead slamming things to 0 with a promise to revisit > it when changing the thread model. Now that we are allowing parallel > transactions, blindly slamming to 1 is no longer a valid option, but > slamming to 0 is overkill when we can instead rely on passthrough to > the plugin's .can_multi_conn. > > [1] https://listman.redhat.com/archives/libguestfs/2019-January/msg00074.html > --- > filters/blocksize/blocksize.c | 42 +++++++++++++---------------------- > 1 file changed, 16 insertions(+), 26 deletions(-) > > diff --git a/filters/blocksize/blocksize.c b/filters/blocksize/blocksize.c > index c3a2d60d..81d0e76c 100644 > --- a/filters/blocksize/blocksize.c > +++ b/filters/blocksize/blocksize.c > @@ -1,5 +1,5 @@ > /* nbdkit > - * Copyright (C) 2018-2020 Red Hat Inc. > + * Copyright (C) 2018-2021 Red Hat Inc. > * > * Redistribution and use in source and binary forms, with or without > * modification, are permitted provided that the following conditions are > @@ -49,23 +49,20 @@ > > #define BLOCKSIZE_MIN_LIMIT (64U * 1024) > > -/* As long as we don't have parallel requests, we can reuse a common > - * buffer for alignment purposes; size it to the maximum we allow for > - * minblock. */ > +/* In order to handle parallel requests safely, this lock must be held > + * when using the bounce buffer. > + */ > +static pthread_mutex_t lock = PTHREAD_MUTEX_INITIALIZER; > + > +/* A single bounce buffer for alignment purposes, guarded by the lock. > + * Size it to the maximum we allow for minblock. > + */ > static char bounce[BLOCKSIZE_MIN_LIMIT]; > + > static unsigned int minblock; > static unsigned int maxdata; > static unsigned int maxlen; > > -/* We are using a common bounce buffer (see above) so we must > - * serialize all requests. > - */ > -static int > -blocksize_thread_model (void) > -{ > - return NBDKIT_THREAD_MODEL_SERIALIZE_ALL_REQUESTS; > -} > - > static int > blocksize_parse (const char *name, const char *s, unsigned int *v) > { > @@ -157,17 +154,6 @@ blocksize_get_size (struct nbdkit_next_ops *next_ops, > void *nxdata, > return ROUND_DOWN (size, minblock); > } > > -static int > -blocksize_can_multi_conn (struct nbdkit_next_ops *next_ops, void *nxdata, > - void *handle) > -{ > - /* Although we are serializing all requests anyway so this likely > - * doesn't make a difference, return false because the bounce buffer > - * is not consistent for flush. > - */ > - return 0; > -} > - > static int > blocksize_pread (struct nbdkit_next_ops *next_ops, void *nxdata, > void *handle, void *b, uint32_t count, uint64_t offs, > @@ -179,6 +165,7 @@ blocksize_pread (struct nbdkit_next_ops *next_ops, void > *nxdata, > > /* Unaligned head */ > if (offs & (minblock - 1)) { > + ACQUIRE_LOCK_FOR_CURRENT_SCOPE (&lock); > drop = offs & (minblock - 1); > keep = MIN (minblock - drop, count); > if (next_ops->pread (nxdata, bounce, minblock, offs - drop, flags, > @@ -202,6 +189,7 @@ blocksize_pread (struct nbdkit_next_ops *next_ops, void > *nxdata, > > /* Unaligned tail */ > if (count) { > + ACQUIRE_LOCK_FOR_CURRENT_SCOPE (&lock); > if (next_ops->pread (nxdata, bounce, minblock, offs, flags, err) == -1) > return -1; > memcpy (buf, bounce, count); > @@ -228,6 +216,7 @@ blocksize_pwrite (struct nbdkit_next_ops *next_ops, void > *nxdata, > > /* Unaligned head */ > if (offs & (minblock - 1)) { > + ACQUIRE_LOCK_FOR_CURRENT_SCOPE (&lock); > drop = offs & (minblock - 1); > keep = MIN (minblock - drop, count); > if (next_ops->pread (nxdata, bounce, minblock, offs - drop, 0, err) == > -1) > @@ -253,6 +242,7 @@ blocksize_pwrite (struct nbdkit_next_ops *next_ops, void > *nxdata, > > /* Unaligned tail */ > if (count) { > + ACQUIRE_LOCK_FOR_CURRENT_SCOPE (&lock); > if (next_ops->pread (nxdata, bounce, minblock, offs, 0, err) == -1) > return -1; > memcpy (bounce, buf, count); > @@ -332,6 +322,7 @@ blocksize_zero (struct nbdkit_next_ops *next_ops, void > *nxdata, > > /* Unaligned head */ > if (offs & (minblock - 1)) { > + ACQUIRE_LOCK_FOR_CURRENT_SCOPE (&lock); > drop = offs & (minblock - 1); > keep = MIN (minblock - drop, count); > if (next_ops->pread (nxdata, bounce, minblock, offs - drop, 0, err) == > -1) > @@ -355,6 +346,7 @@ blocksize_zero (struct nbdkit_next_ops *next_ops, void > *nxdata, > > /* Unaligned tail */ > if (count) { > + ACQUIRE_LOCK_FOR_CURRENT_SCOPE (&lock); > if (next_ops->pread (nxdata, bounce, minblock, offs, 0, err) == -1) > return -1; > memset (bounce, 0, count); > @@ -437,12 +429,10 @@ blocksize_cache (struct nbdkit_next_ops *next_ops, void > *nxdata, > static struct nbdkit_filter filter = { > .name = "blocksize", > .longname = "nbdkit blocksize filter", > - .thread_model = blocksize_thread_model, > .config = blocksize_config, > .config_complete = blocksize_config_complete, > .config_help = blocksize_config_help, > .get_size = blocksize_get_size, > - .can_multi_conn = blocksize_can_multi_conn, > .pread = blocksize_pread, > .pwrite = blocksize_pwrite, > .trim = blocksize_trim, > --
ACK Rich. -- Richard Jones, Virtualization Group, Red Hat http://people.redhat.com/~rjones Read my programming and virtualization blog: http://rwmj.wordpress.com virt-top is 'top' for virtual machines. Tiny program with many powerful monitoring features, net stats, disk stats, logging, etc. http://people.redhat.com/~rjones/virt-top _______________________________________________ Libguestfs mailing list [email protected] https://listman.redhat.com/mailman/listinfo/libguestfs
